I. Field of the Invention
The present invention relates to a pleochroic or dichroic dye which is to be dissolved in a liquid crystal for use in a liquid crystal device utilizing the guest-host effect such as a liquid crystal display device or a liquid crystal light valve.
II. Description of the Prior Art
In general, it is desired that a liquid crystal device utilizing the guest-host effect, such as a liquid crystal display device, provide a display of good contrast and have a short response time. Furthermore, it is also desirable that the display color not fade over a long period of time.
The display contrast of a guest-host effect liquid crystal display device is mainly determined by the dichroic ratio of a dichroic dye dissolved in the liquid crystal. The higher the dichroic ratio, the better the contrast. However, even if the dichroic ratio of the dye is high, the display color becomes lighter and a satisfactory display cannot be obtained if the coloring power (mole absorptivity) of the dye itself is low. If the coloring power of the dye is low, the amount of the dye added in the liquid crystal must be increased to obtain a darker display. However, this adversely affects the liquid crystal properties of the liquid crystal molecules and prolongs the response time upon application of a driving voltage. Furthermore, the dichroic dye must have satisfactory compatibility with the liquid crystal. If the dichroic dye has a poor compatibility with the liquid crystal, the dye may precipitate out of the liquid crystal. This precipitation or segregation of the dye is particularly notable at low temperatures. The precipitation causes color irregularities and lowers the quality of the display product.
In order to obtain a desirable guest-host effect liquid crystal device of the type described above, the guest dichroic dye to be dissolved in the host liquid crystal must satisfy the following conditions:
1. The dye must have a high dichroic ratio.
2. The dye must have a high coloring power and be able to provide a dark display color when only a small amount is added in the host liquid crystal.
3. The dye must have an excellent compatibility with the liquid crystal and must not precipitate at low temperatures.
Conventional dichroic dyes include benzene-based azo dyes, stilbene-based dyes, and anthraquinone-based dyes. However, none of these dyes satisfy all these three requirements.